The overall objective of Project 1 is to induce durable tolerance across a pig-to-baboon xenogeneic barrier through transplantation of vascularized thymic grafts. For this purpose, we will: 1) optimize our immunosuppressive drug regimen: We have demonstrated previously that vascularized thymic grafts support thymopoiesis and induce transplant tolerance across fully mismatched allogeneic barriers. In this xenogeneic model, we will attempt to optimize the immunomodulatory treatment regimen required to achieve durable thymus engraftment; 2) determine whether the engrafted thymus can induce tolerance to a kidney xenograftfrom an inbred donor animal genetically identical to the thymus donor. If successful, the strategy will be extended to simultaneous thymus plus kidney xenotransplantation; and 3) elucidate the mechanism of donor thymus-induced tolerance, using in vitro assays and focusing on the specificity of tolerance across this species barrier. These studies will be highly interactive with other projects in this PPG. Thus, the optimal treatment regimen will be modified, if necessary, on the basis of results from a mouse model of pig-to-primate tolerance through thymus transplantation being investigated by Sykes and colleagues in Project 3. If eventual antibody-mediated rejection cannot be effectively prevented through thymic transplantation, we will attempt to induce B-cell tolerance by combining thymic transplantation with hematopoietic transplantation, being studied in Project 2. We will also collaborate with Project 5 on the mechanism of any post-transplant coagulation disorders observed. Together, we hope these studies will provide new insights into the potential role of thymic transplantation in the induction of transplantation tolerance.